As people peered out of the doorways, the volunteers turned several corners and stopped in front of a stairwell door. One of them pushed it open.
There stood an office worker, pressing a cellphone to one ear and holding a lit cigarette in his other hand. Someone had turned him in.
A stern lecture followed from the group's leader, a stocky, 32-year-old fine arts teacher named Liu Li.
"Today, we won't punish you, but we will criticize and educate you," Liu said in a carrying voice, as the worker bowed and apologized repeatedly. "Don't throw cigarette butts around. You must not act like this next time."
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Zealous volunteers and anti-smoking advocates have made some headway against millions of occasionally intransigent smokers and the state-run cigarette monopoly, a large and powerful force in China's government and economy.
Cigarettes are a cultural symbol in China, where national leaders dating back to Mao Zedong were well-known smokers, and where cigarettes are still handed out commonly at weddings, banquets and holiday celebrations.
The tobacco industry employs more than 300,000 people and remains a key source of revenue in the national budget. The State Tobacco Monopoly Administration generated more than $150 billion in tax revenues just last year alone.
China has more than 300 million smokers and nearly half of China's adult males smoke regularly, according to the World Bank.
For all of the attention given to China's notorious air pollution, it's smoking that's often far more damaging and far easier to correct, said Dr. Bernhard Schwartlander, who has worked for several years in China as the World Health Organisation's local representative.